The present invention relates to a long-haul and large-capacity optical transmission system used in a long trunk line having large traffic capacity, in particular, relates to such a system which uses optical intensity modulation in a transmitter side.
In a simple optical transmission system, an optical intensity modulation system in transmitter side is used, and a direct detection system is used in receiver side. The direct detection system detects a received signal without using interference of coherent light from other source. That system is abbreviated to IM-DD system. Two modulation methods have been known in optical intensity modulation. One of them is a direct modulation in which an intensity of a semiconductor laser output is directly controlled, and the other is an external modulation in which an output beam of a semiconductor laser is modulated by using an optical intensity modulator.
A direct modulation system has the disadvantage of undue expand of spectrum because of undesirable frequency modulation which coincides with optical intensity modulation which is called chirping, and transmission quality is deteriorated due to chromatic dispersion of an optical fiber.
As for an external modulation system, many types of modulators have been proposed, and among them, a Mach-Zender interferometer type (MZ type) modulator has been widely used for high-speed and long-haul transmission systems, since it avoids essentially chirping.
FIG. 13 shows a prior optical transmission system using a MZ type optical intensity modulator in a transmitter. In the figure, a MZ type optical intensity modulator 70 has a coupler 71 dividing a light beam to a pair of branch waveguides 72a and 72b which provide a phase difference between light beams in those waveguides, and a coupler 73 which combines outputs of those waveguides so that optical intensity modulation is carried out. Optical intensity of the combiner output is high if phases in two waveguides 72a and 72b coincides with each other, and it is low if phases in two waveguides are opposite with each other. In a MZ type optical intensity modulator, transmittance or intensity of output light depends upon said phase difference which is controlled by potential or voltage applied to electrodes 74a and 74b along said branch waveguides, as shown in FIG. 14. In a prior binary IM-DD system, each of binary values (0 and 1) is assigned to the maximum transmittance point A and the minimum transmittance point B. In the configuration of FIG. 13, a binary input signal is separated to two signals with one signal applied to an inverter 75 so that a pair of complementary signals are obtained, so that those complementary signals are applied to electrodes 74a and 74b each related to branched waveguides 72a and 72b . In the above configuration, MZ type modulator operates in push-pull operation, so that undesirable chirping is completely removed (see F. Koyama and K. Iga, IEEE J. Lightwave Technol., vol.6, No. 1, pages 87-93, 1988).
An output beam of a semiconductor laser 76 in FIG. 13 is modulated by an MZ type modulator 70 which is driven by a pair of complementary signals, and modulated beam is forwarded to an optical transmission cable 77. In a receiver side, a modulated beam at output of the cable 77 is directly detected by an optical detection circuit 78. An output of the optical detection circuit 78 is decided by a decision circuit 79 to provide demodulated data signal.
It should be noted that an optical binary intensity modulation signal has spectrum as shown in FIG. 15, in which a large carrier frequency component exists, and spectrum expands up to twice of bit rate on both sides of carrier frequency. In FIG. 15, horizontal axis shows frequency with 1.2 GHz for each scale, and vertical axis shows power with 5 dB for each scale.
Said carrier frequency component is undesirable because of deterioration of transmission quality due to non-linear characteristics of an optical fiber, in particular, restriction of input power into an optical fiber due to stimulated Brillouin scattering (see T. Sugie, IEEE J. Lightwave Technol., vol.9, pages 1145-1155, 1991). Further, it causes the increase of crosstalk due to four-wave mixing in an optical wavelength-division multiplexed transmission system (see N. Shibata et al., IEEE J. Quantum Electron., vol. QE-23, pages 1205-1210, 1987). Further, the undue expansion of signal spectrum causes the deterioration of receiver sensitivity in long-haul and larvae capacity transmission system due to chromatic dispersion of an optical fiber, and/or the decrease of frequency utilizing efficiency due to crosstalk between optical wavelength channels. The effect of chromatic dispersion is not negligible in some channels because of dispersion slope even when disperison-shifted fiber is used in a long transmission line for wavelength-division multiplexed high-speed signal. Those problems restrict transmission distance, transmission rate, and/or trans, mission capacity, and therefore, those problems must be solved in developing an optical network.
For extending the transmission distance limited by chromatic dispersion, one solution for suppressing undue expansion of spectrum of an optical intensity modulation signal is the use of duobinary signal (see X.Gu and L. C. Blank, Electron. Lett. vol. 29, No. 25, pages 2209-2211, 1993).
FIG. 16 shows a prior optical transmission system using duobinary signal. In the figure, a binary data signal is converted to duobinary signal with an encoding circuit 80, which has a one-bit delay line (T) 81 and an exclusive-OR circuit (EXOR) 82 for differential encoding to provide a precoded sequence, and a low pass filter 87 for providing a duobinary signal from said precoded sequence. The low pass filter 87 for providing a duobinary signal doubles as a bandwidth restriction filter. An optical intensity modulator 85 modulates an optical carrier from the semiconductor laser with said duobinary signal which is an output of the encoding circuit 80, so that three-level intensity modulation signal is launched into an optical fiber cable 77. In a receiver side, an output signal of the optical fiber cable 77 is directly detected by an optical detection circuit 78. The output of the optical detection circuit is three-level signal, which is converted to binary signal by using a pair of decision circuits 79a and 79b, and original binary signal is recovered by EX-OR circuit 79c.
An optical transmission system using duobinary signal as shown in FIG. 16 has advantage that transmission quality is less deteriorated due to chromatic dispersion of an optical fiber, since spectrum bandwidth of optical signal is narrow. The effectiveness of that system has been confirmed in a 10 Gbit/s, 100 km transmission experiment using an MZ type optical intensity modulator (see X. Gu and L. C. Blank, Electron. Lett. vol. 29, No. 25, pages 2209-2211, 1993).
However, a prior optical three-level transmission system using duobinary code has the disadvantage that receiver sensitivity is degraded by approximate 3 dB as compared with that of a binary IM-DD transmission system, because of decrease of distance between signal points to be separated, since an optical signal is converted to electrical three-level signal (see X. Gu and L. C. Blank, electron. Lett. vol. 29, No. 25, pages 2209-2211, 1993). (Three values of a duobinary signals are assigned to the points A, B and C in FIG. 14, and so, the transmittance at the point C is half of that at point A). Further, in a receiver side, two decision circuits and an EX-OR circuit must be installed for recovering original binary signal from the detected three-level signal, and therefore the configuration of a receiver is complicated. Further, the spectrum has still carrier frequency component, as is the case of a binary intensity modulated signal, therefore, it has the disadvantages of restriction of input power into an optical fiber due to stimulated Brillouin scattering, and/or crosstalk due to four-wave mixing.